So...I have this little theory that Stan Lee actually created the Mighty Thor as a direct way to compete with Superman.

"What madness does yon newcomer speak?!"

I'm glad you asked.

When I was in grade school, I purchased a copy of Origins of Marvel Comics, by Stan Lee. In that particular book, Stan Lee discusses the origins of some of Marvel's most popular characters, The Fantastic Four, The Incredible Hulk, The Amazing Spider-Man, The Mighty Thor and Dr Strange, master of the mystical arts. However, in a few of his prologues, Stan brings up National Comics (now DC comics) and the character of Superman. It seems possible to me from reading those passages and having bought the book again and re-reading it, that Stan wanted to compete with Superman. For example, on page 74 in his prologue to the Incredible Hulk, Stan makes this comment: "I had pretty much decided to let our second Marvel-style magazine feature someone with superhuman strength. But there had been, and still were, many such characters at that time, with National Comics' Superman as the first that comes to mind. Certainly, there would be nothing terribly original about someone who had the strength of Superman. But that's where the fun came in. It would be my job to take a cliche' concept and make it seem new and fresh, exciting and relevant."
Now, I mean really...that comment could be interpreted in several different ways, right? There. I said it. But it's safe to say, Superman was at least a passing thought in creating the Hulk, right?
Let's continue. On page 133, in his prologue to the Amazing Spider-Man, Stan had this to say, "I can still remember my sinister little scheme with Martin Goodman. I told him I would try to do the whole new strip in a tongue-in-cheek manner. Everybody knew about Superman-so the time had come for a competitor to make the scene; and what fun it would be to call him Spider-Man." Of course, this comment is made in retrospect...I mean did Stan really think he was going to be able to compete with Superman in terms of sales when he and Ditko came up with the idea of Spider-Man? Methinks not! But it is interesting that his comments are an afterthought...
Again, Superman is mentioned...but this time, Stan mentions directly competing with Superman in terms of sales. Previously, he mentioned someone with Superman's strength, but making the concept fresh and new. With Spider-Man, he mentions competing in terms of sales. Notice Stan's thoughts in what inspired his creation for his next character, in his prologue to the Mighty Thor , Meanwhile, back in Asgard...and notice that he had not come up with who the character would be before he decided his nature and set of powers (he was already headed in a very specific direction):
"Look at it this way: Suppose you had a newly created stable of superstars which consisted of a teenager who could burst into flame and fly through the air, a stretchable scientist with skin like Silly Putty, his ofttimes invisible lady love, and a multimuscled misfit with lumpy orange skin-to say nothing of a wall -crawling Wunderkind and a jolly green giant-what in the name of comicdom assembled would you do for an encore? Sure, we were always striving for variety, but now it was getting ridiculous." He continues two paragraphs down:
"But what was left to invent? Who could be stronger than The Hulk? Who could be smarter than Mr Fantastic? We already had a kid who could fly, one who could walk on walls and ceilings, and a female who could fade away whenever danger threatened-or whenever the artist ran out of ink. As you can see, we were hooked onsuperlatives at that time, always trying to come up with characters who were bigger, better, stronger. However, we had painted ourselves into a corner. The only one who could top the heroes we already had would Super-God, but I didn't think the world was quite ready for that concept just yet. So, it was back to the ol' drawing board."
"I must have gone through a dozen pencils and a thousand sheets of paper in the days that followed, making notes and sketches, listing names and titles, and jotting down every type of superpower I could think of. But I kept coming back to the same ludicrous idea: the only way to top the others would be with Super-God."
Stan goes onto to say that during a radio show, the host referred to his stories as ,"...a twentieth century mythology." And you guys know where this is going.
Notice his comments taken from page 181 of the book:
"Then, another thought hit me. I wanted him to be able to fly. I wanted him to be able to zip around the sky and make the trip between heaven and earth without waiting for Pan Am. The Hulk simulated flight by leaping into the air; the Human Torch did it by bursting into flame; Spidey had his webbing and swing around like Tarzan. God only knows how Superman manages it-I never figured that out. I didn't want to merely say, 'There goes our hero, flying off again.' I wanted it to be somehow believable." (Stan's striving for suspension of disbelief used to be a Marvel trademark. Quesada has on the other hand said, "It's comics...anything goes!")
Here's a funny excerpt I found on you tube: